Perforin-mediated cytolysis is essential for clearance of lymphocytic choriomeningitis virus (LCMV) and perforin-deficient mice are models for familial hemophagocytic lymphohistiocytosis (FHL), a uniformly fatal disease associated with viral infection in individuals with mutations in perforin. In addition, growing evidence suggests that perforin regulates multiple aspects of CD8+ T cell homeostasis after infection. We recently, showed that naive perforin-deficient mice (BALB/c background) survive LCMV infection; however, if the mice contain LCMV-specific memory CD8+ T cells, they rapidly succumb to LCMV. Mortality is associated with massive expansion of memory CD8+ T cells, which exhibit aberrant regulation of IFN-gamma and TNF. Depletion of CD8+ T cells prior to LCMV infection or treatment with anti-IFN-gamma antibody prevents mortality. Thus, prior vaccination converts a non-lethal persistent infection into a rapidly fatal disease mediated by CD8+ T cells in the absence of perforin. We wish to understand the basic virus and CD8+ T cell interactions in the absence of perforin that push the immune system to such a drastic and ultimately fatal response. Results from the proposed studies may shed light on the pathogenesis of FHL and also identify beneficial therapeutic interventions. Finally, agents that cause viral hemorrhagic fevers, including some LCMV strains, are CDC class A biodefense agents. It is likely that much effort will be placed on developing vaccines for these agents. Our studies suggest that some individuals may be harmed rather than helped by vaccination. Thus, we will determine the specifics of vaccine-associated mortality in the LCMV model and whether mortality in vaccinated perforin-deficient hosts extend to vaccinia virus or to other immunocompromised hosts. Our long-term goal is to understand why vaccination leads to mortality in immunocompromised hosts so that this outcome can be minimized through rational vaccine design. We will pursue this goal through the following specific aims. Aim 1. Determine why perforin-deficiency results in massive CDS+ T cell expansion after LCMV infection of vaccinated perforin-deficient (PKO) mice. Aim 2. Determine why perforin-deficiency results in CD8+ T cell cytokine dysregulation and mortality after LCMV infection of vaccinated PKO mice. Aim 3. Analyze CD8+ T cell and pathogen parameters that determine the outcome of LCMV infection in vaccinated PKO mice. Aim 4. Determine if vaccination-associated mortality extends to other immunodeficiencies or pathogens.